The invention relates to polycrystalline diamond abrasive compacts and a method of producing polycrystalline diamond abrasive compacts.
Polycrystalline diamond abrasive compacts (PDC) are used extensively in cutting, milling, grinding, drilling and other abrasive operations due to the high abrasion resistance of the polycrystalline diamond component. In particular, they find use as shear cutting elements included in drilling bits used for subterranean drilling. A commonly used PDC is one that comprises a layer of coherently bonded diamond particles or polycrystalline diamond (PCD) bonded to a substrate. The diamond particle content of these layers is typically high and there is generally an extensive amount of direct diamond-to-diamond bonding or contact. Diamond compacts are generally sintered under elevated temperature and pressure conditions at which the diamond particles are crystallographically or thermodynamically stable.
Examples of composite abrasive compacts can be found described in U.S. Pat. Nos. 3,745,623; 3,767,371 and 3,743,489.
The PCD layer tends to be relatively brittle and this often limits the lifespan of the tool in application. Hence the PCD layer is generally bonded to a metal backing material, serving as a hard-wearing support for the diamond composite portion. By far the most common form of the resultant body is a disc of polycrystalline diamond bonded to a cylinder of cemented carbide such as WC—Co. Bonding of these two elements is usually achieved in-situ during the sintering of the diamond powder precursor at high pressure and temperature (HpHT).
The PCD layer of this type of abrasive compact will typically contain a catalyst/solvent or binder phase in addition to the diamond particles. This typically takes the form of a metal binder matrix which is intermingled with the intergrown network of particulate diamond material. This matrix usually comprises a metal exhibiting catalytic or solvating activity towards carbon such as cobalt, nickel, iron or an alloy containing one or more such metals.
The matrix or binder phase may also contain additional phases. In typical abrasive compacts of the type of this invention, these will constitute less than 10 mass % of the final binder phase. These may take the form of additional separate phases such as metal carbides which are then embedded in the softer metallic matrix, or they may take the form of elements in alloyed form within the dominant metal phase
Composite abrasive compacts are generally produced by placing the components necessary to form an abrasive compact, in particulate form, on a cemented carbide substrate. The components may, in addition to ultrahard particles, comprise solvent/catalyst powder, sintering or binder aid material. This unbonded assembly is placed in a reaction capsule which is then placed in the reaction zone of a conventional high pressure/high temperature apparatus. The contents of the reaction capsule are then subjected to suitable conditions of elevated temperature and pressure to enable sintering of the overall structure to occur.
It is common practice to rely at least partially on binder originating from the cemented carbide as a source of metallic binder material for the sintered polycrystalline diamond. (In many cases however, additional metal binder powder is admixed with the diamond powder before sintering.) This binder phase metal then functions as the liquid-phase medium for promoting the sintering of the diamond portion under the imposed sintering conditions.
Under typical high pressure, high temperature sintering conditions, binder metal phase originating from the cemented carbide substrate will also carry with it appreciable levels of dissolved species originating from the carbide layer, as it infiltrates the diamond layer. The amount of dissolved species is strongly affected by the pressure and temperature conditions of sintering—where higher temperatures will typically increase the amount in solution. When the preferred substrate of WC—Co is used, these are W-based species.
As it infiltrates into the PCD region, this dissolved tungsten reacts with solvent metal and carbon from the diamond layer, and may precipitate out carbide-based phases. In some cases, depending on the nature of the metallurgy of the binder phase, so-called eta phase will also form.
Eta-phase is well-known in the general carbide industry; and is taken to mean compositions of W, C and solvent metal, M (in this case, cobalt) such as WxMyC etc. One of these, an intermetallic carbide, specifically Co3W3C, remains in the final ultrahard compact if it forms. This phase is known to be brittle and can provide sites for crack initiation and propagation in the final composite structure. Its presence can hence result in a deterioration in composite properties.
The prior art for carbide manufacture contains several references to methods for controlling and/or manipulating the formation of eta-phase in conventional carbide materials. For example, U.S. Pat Application 2005/0061105, which issued as U.S. Pat. No. 6,869,460, discusses a method for achieving an eta-phase free carbide composite by manipulating the binder concentration in the material.
Eta-phase, Co3W3C, will typically be present in polycrystalline diamond abrasive compacts where significant amounts of dissolved W have been carried up from the substrate on infiltration. They hence occur in conjunction with the formation of other precipitating W-based phases such as WC in the PCD layer. Eta-phase appears to be particularly observed where relatively higher sintering temperatures have been utilised to improve diamond-to-diamond sinter quality. At lower sintering temperatures, eta-phase can be reduced; however, reducing sinter temperature is not practicable as this will typically result in sub-optimal sintering conditions and hence a less desirable PCD.
The development of an abrasive compact that can achieve optimal properties of impact and wear resistance in the PCD layer is highly desirable. The difficulty lies in that these optimal properties typically occur in a similar sintering environment to that where carbide-based defect phases in the PCD layer can arise. These defect phases themselves have a highly detrimental effect on these same required properties. Hence a means of preventing or inhibiting their formation is highly desirable.